Minimum width of cavity 2021

[Markbon]

Hello. I wonder if anyone can bring me up to date on the regs for cavity walls (block and block). I've been out of the business for some time. Ive heard there's a change to the rules, or one is pending? But I cant find a recent part L by googling it.

Back in the day you could reduce the cavity if you used certain blocks and insulation. I'd appreciate if anyone knows if there's still a way of doing that.
Thanks.

 

[^woody^]

There is no minimum. It's the whole wall u value that matters.

 

[Markbon]

There is no minimum. It's the whole wall u value that matters.

Thanks.

Does anyone know what the u value is and how its possible to get it?
I think ive found online that its 0.29 and thats possible with 100mm of dritherm but that could be wrong.
If its still possible to do it with 100mm cavity thats not too bad. I'd like find out what the smallest way of doing it is if I can.

 

[DevilDamo]

https://lmgtfy.app/?q=approved+document+l

 

[johnny2007]

As said, there's no minimum, you could even build a solid 9 or 12 inch wall if you wish.
I understand you're trying to find a compromise.
I've built porches with a 50mm cavity insulated with rockwool and they stay dry and reasonably warm.
If it's a new extension you will need to satisfy BC that it reaches the required u value.

 

[^woody^]

It's 0.28 for extension walls, and that will generally mean a wall between 280 and 300mm, however you try to jig the blocks and insulation about. However, I recall a post in this or the building forum where a rigid board requiring no cavity was mentioned, which might reduce things a little.

Otherwise, a thinner wall can be obtained with a timber frame, and if there is no masonry external skin then this could be less than 200mm thick